The Lewis basicity of nitrido complexes. Theoretical investigation of the structure and bonding of Cl2 (PH3)3ReN–X (X = BH3, BCl3, BBr3, AlH3, AlCl3, AlBr3, GaH3, GaCl3, GaBr3, O, S, Se, Te)

Abstract.

Quantum chemical calculations using gradient-corrected density functional theory (B3LYP) and ab initio methods at the MP2 level are reported for the geometries and bond energies of the nitrido complexes Cl₂ (PH₃)₃ReN–X (X = BH₃, BCl₃, BBr₃, AlH₃, AlCl₃, AlBr₃, GaH₃, GaCl₃, GaBr₃, O, S, Se, Te). The theoretical geometries are in excellent agreement with experimental values of related complexes which have larger phosphine ligands. The parent nitrido complex Cl₂(PH₃)₃ReN is a very strong Lewis base. The calculated bond dissociation energy of Cl₂(PH₃)₃ReN–AlCl₃ is D _e = 43.7 kcal/mol, which is nearly as high as the bond energy of Me₃N–AlCl₃. The donor-acceptor bonds of the other Cl₂(PH₃)₃ReN–AY₃ complexes are also very strong. Even stronger N–X bonds are predicted for most of the nitrido-chalcogen complexes, which exhibit the trend X = O ≫ S > Se > Te. Analysis of the electronic structure shows that the parent compound Cl₂(PH₃)₃ReN has a Re–N triple bond. The Re–N σ bond is clearly polarized towards nitrogen, while the two π bonds are nearly nonpolar. The Re–N σ and π bonds become more polarized toward nitrogen when a Lewis acid or a chalcogen atom is attached. Bonding in AY₃ complexes should be described as Cl₂(PH₃)₃ReE≡N→AY₃, while the chalcogen complexes should be written with double bonds Cl₂(PH₃)₃Re=N=X. The charge-decomposition analysis indicates that the nitrogen-chalcogen bonds of the heavier chalcogen complexes with X = S, Se, Te can also be interpreted as donor-acceptor bonds between the nitrido complex acting as a Lewis base and the chalcogen atom with an empty p(σ) orbital acting as a Lewis acid. The nitrido oxo complex Cl₂(PH₃)₃ Re=N=O has a covalent N–O double bond.